Experimental and Theoretical Evidence for a Concerted Catalysis by Water Clusters in the Hydrolysis of Isocyanates

Abstract

A kinetic and mechanistic investigation of the catalyzed hydration of isocyanates was undertaken. Both experimental and theoretical results showed that the hydrolysis reaction involves a chain of water molecules. The detailed hydration mechanism by water and water clusters (H-N=C=O + n(H(2)O) --> H(2)NCOOH + (n - 1)H(2)O, n = 1-3) has been modeled by ab initio methods, both in the gas phase and in aqueous solution. While two water molecules in the form of a dimer seem to play the key role in hydrating the isocyanate, a third water molecule may be needed to bridge the gap from the point of attack on the isocyanate to the water dimer and to facilitate further the hydration. In accordance with these facts, experimental results imply a second-order dependence on water during its nucleophilic addition to phenyl isocyanate, over a wide concentration range. In this specific case, water oligomers higher than the dimer seem to make no appreciable contribution to the rate of the hydrolysis reaction. The nucleophilic addition occurs in a concerted way across the N=C bond of the isocyanate rather than across the C=O bond. This preferential reaction mechanism could be rationalized in terms of Fukui functions for both nucleophilic and electrophilic attacks. Although a charge separation occurs in the transition state, electrostatic solvent effects are not quite important in reducing only marginally the energy barriers.

A comprehensive theoretical study on the hydrolysis of carbonyl sulfide in the neutral water

“…There are many theoretical studies of the hydration of cumulenes, including CO 2 , [20][21][22][23][24] H 2 C¼ ¼C¼ ¼O, [25][26][27] CH 2 ¼ ¼C¼ ¼NH, 28 HN¼ ¼C¼ ¼O, 29 and HN¼ ¼C¼ ¼NH, [30][31][32] showing that consideration of a second water molecule resulted in catalysis effect with respect to the single water molecule hydrolysis; similar results were also found for the hydrolysis of SO 3 (ref. 33) and SO 2 .…”

Use of Local Softness for the Interpretation of Reaction Mechanisms

“…The most interesting result for this type of addition is perhaps the finding that the most kinetically favored channel does not always lead to the most thermodynamically stable adduct. Formation of novel reaction intermediates, namely the less stable enols, was frequently predicted, and this theoretical prediction was finally verified by experimental evidence [20,21,24,25].…”

“…The hydration occurs by an initial nucleophilic attack of one water oxygen to one site of the double bond, followed by a proton relay within the water chain and ended up by an electrophilic attack of a proton to another site [19][20][21][22][23].…”

“…We have considered the mechanism of hydration [19][20][21][22][23] and similar alcololysis [24] and amination [25] reactions in several typical cumulenes such as isocyanates (R-C=N=O), ketenes (RRC=C=O), ketenimines (RRC=C=NR), carbodiimides (RN=C=NR), carbon dioxide (O=C=O) and carbon suboxide (O=C=C=C=O). The most interesting result for this type of addition is perhaps the finding that the most kinetically favored channel does not always lead to the most thermodynamically stable adduct.…”

Chemical carcinogens in non-enzymatic cytosine deamination: 3-isocyanatoacrylonitrile

“…Ubiquitous water may react with 7 by addition, substitution (R≠H), or deprotonation (R=H). Water addition forms of carbamic acid [10] and subsequent decarboxylation [11] leads to (Z)-3-aminoacrylonitrile 9. (Z)-3-aminoacrylonitrile is susceptible to base- [12,13] and acid-catalyzed [14] nucleophilic addition to the C=C and C≡N bonds and this chemistry also might lead to DNA adducts.…”